The present invention relates generally to laser eye surgery methods and systems. More specifically, the present invention relates to registering a first image of a patient's eye with a second image of a patients eye and to tracking a position and a torsional orientation of the patient's eye during laser eye surgery so as to register a customized ablation profile with the patient's eye.
Known laser eye procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye to alter the refractive characteristics of the eye. The laser removes a selected shape of the corneal tissue, often to correct refractive errors of the eye. Ultraviolet laser ablation results in photo-decomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photochemically, directly breaking the intermolecular bonds.
Laser ablation procedures can remove the targeted stroma of the cornea to change the cornea's contour for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and the like. Control over the distribution of ablation energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, fixed and moveable apertures, controlled scanning systems, eye movement tracking mechanisms, and the like. In known systems, the laser beam often comprises a series of discrete pulses of laser light energy, with the total shape and amount of tissue removed being determined by the shape, size, location, and/or number of a pattern of laser energy pulses impinging on the cornea. A variety of algorithms may be used to calculate the pattern of laser pulses used to reshape the cornea so as to correct a refractive error of the eye. Known systems make use of a variety of forms of lasers and/or laser energy to effect the correction, including infrared lasers, ultraviolet lasers, femtosecond lasers, wavelength multiplied solid-state lasers, and the like. Alternative vision correction techniques make use of radial incisions in the cornea, intraocular lenses, removable corneal support structures, thermal shaping, and the like.
Known corneal correction treatment methods have generally been successful in correcting standard vision errors, such as myopia, hyperopia, astigmatism, and the like. However, as with all successes, still further improvements would be desirable. Toward that end, wavefront measurement systems are now available to measure the refractive characteristics of a particular patient's eye. By customizing an ablation pattern based on wavefront measurements, it may be possible to correct minor refractive errors so as to reliably and repeatably provide visual acuities greater than 20/20. Alternatively, it may be desirable to correct aberrations of the eye that reduce visual acuity to less than 20/20. Unfortunately, these measurement systems are not immune from measurement error. Similarly, the calculation of the ablation profile, the transfer of information from the measurement system to the ablation system, and the operation of the ablation system all provide opportunities for the introduction of errors, so that the actual visual acuities provided by real-world wavefront-based correction systems may not be as good as might be theoretically possible.
One potential problem with the use of wavefront measurements is aligning the customized laser ablation pattern with the patient's eye. In order to achieve precise registration between the wavefront measurement and the treatment to be delivered to the patient's eye, the wavefront measurement and the eye should share a common coordinate system. For example, when the wavefront measurement is taken, the patient will generally be in a seated position. However, when the laser eye surgery is being performed, the patient will generally be in a supine position, which may not position the patient's eye in the same position or torsional orientation as the eye when the wavefront measurement was taken.
Moreover, even if the patient is positioned in the same initial position and/or torsional orientation, the eye often undergoes a cyclotorsional rotation. If this rotation is not properly accounted for, the benefits of the refractive surgery would be reduced, particularly in cases of astigmatism and other non-rotationally symmetric aberrations. It has been reported by numerous investigators and researchers that human eyes may undergo torsional movements, usually within 15 degrees from the resting position, but typically it is around 2 to 7 degrees around their axes, during normal activities. The amount of rotation depends on the individual, the stimulus being viewed, and it may depend on the motion and orientation of the person's head and body. Such torsional movement of the patient's eye during the ablation may cause a non-optimal delivery of the customized ablation pattern to the patient's eye, particularly in cases of astigmatism and other-non-rotationally symmetric aberrations.
In light of the above, it would be desirable to provide methods and devices which can accurately register the patient's eye with the customized ablation pattern. Additionally, it would be desirable to account for the positional movement and torsional rotation of the patient's eyes during a laser surgery procedure.